1. Field
Embodiments of the present invention relate to a binder composition for a rechargeable battery and a rechargeable battery having the same.
2. Description of the Related Art
Lithium batteries having high voltage and high energy density are used in a wide variety of applications. For example, lithium batteries can be used to power electric vehicles. Here, since the electric vehicles, such as hybrid electric vehicles (HEV) or plug-in hybrid electric vehicles (PHEVs), are usually driven at high temperature, are charged or discharged with a large amount of electricity, and are used for an extended period of time, the lithium batteries having excellent discharge capacity and lifetime characteristics are desired.
A carbon-based material is a porous material and is stable due to a little volumetric change during charge and discharge. However, the carbon-based material generally has a low capacity due to a porous carbon structure. For example, a theoretic capacity of graphite having high crystallinity is 372 mAh/g in a LiC6.
As the negative electrode active material having higher electric capacity than the carbon-based material, a metal alloyable with lithium can be used. For example, the metal alloyable with lithium may include Si, Sn, Al, etc. However, since the metal alloyable with lithium is prone to deteriorate, it has poor lifetime characteristics. For example, as charge and discharge cycles are repeated, aggregation and pulverization of Sn particles are repeated, thereby making the Sn particles electrically disconnected.
As the binder for a lithium battery capable of suppressing expansion of an electrode, polyimide, polyamideimide, etc. have been proposed. In a case of a lithium polymer battery requiring rolling and pressing of the electrode to cure the electrode, the electrode is prone to cracks, making it difficult to achieve commercialization.
As the binder for a lithium battery, a diene-based copolymer may also be used in a negative electrode. The diene-based copolymer binder has high flexibility but low strength when it is immersed in an electrolytic solution. Therefore, when a non-carbon based high capacity negative electrode active material, such as Si or Sn, which is capable of forming an alloy with lithium, is used, it is difficult to suppress expansion of the electrode.
In addition, the diene-based copolymer binder is prepared by polymerization methods, including emulsion polymerization, suspension polymerization, and so on. For example, processes for making a 2-phase (core/shell) particle structure in emulsion polymerization or continuously converting polymer composition by power-feed polymerization have been introduced. However, the polymer binder having the core/shell particle structure or the continuously converted polymer composition is disadvantageous in that flexibility and balanced strength after it is immersed in an electrolytic solution is difficult to achieve.
To solve the disadvantages stated above, there has been a demand for materials of a negative electrode, including a negative electrode active material, a binder, and so on, which has excellent storage modulus to control a high shrinkage-expansion ratio of the negative electrode active material. In particular, there are continuing research efforts to develop a binder capable of preventing electrode deformation due to expansion of the negative electrode active material by improving adhesion between the negative electrode active material and a current collector. To overcome limits of the comparable technology, it is desired to develop a binder having a sufficiently high storage modulus to improve lifetime characteristics of a lithium battery by coping with and/or suppressing a volumetric change of the non-carbon-based negative electrode active material.